This invention relates to a motor speed control circuit and a control circuit for reducing the energy consumption of induction motors running under no load or light loading conditions, and more particularly to such control circuits utilizing a triac whose conduction angle is varied as a function of the charge appearing during each half cycle on a large capacitor in series with the motor, said charge therefore depending on the torque load being applied to the motor which is being controlled.
The characteristics of triacs, which in effect are controlled bi-directional current-conducting semiconductor, makes such devices particularly suitable for controlling power supply circuits which operate on full-wave alternating current sources. Triacs have a control electrode or gate which has a triggering circuit such as a diac coupled thereto for controlling the firing and direction of current flow through the triac.
Although triacs are suitable for motor speed control, the prior art circuits in which triacs have functioned have been complex and somewhat difficult to control over wide ranges of operating speeds. The prior art motor speed control circuits exhibit serious shortcomings in those applications where it is desirable to maintain a set speed under variable load conditions. For example, consider a hand-held electric drill. When the drill is pressed into a workpiece, the motor is subjected to an increased torque load, and when the pressure on the drill is decreased, the motor experiences a decrease in torque load. Accordingly, it is common experience today that the motor is a commercially available prior art hand-held electric drill will slow down for a given trigger setting when the torque load is increased, and vice-versa. Of course, the speed of such a drill may be increased by manually increasing the trigger setting, but this change in trigger setting may result in excessive drill speed for the particular drilling job as soon as the drill pressure is reduced. Moreover, it is a bother and a distraction to the user to keep changing the trigger setting in an effort to make the motor speed behave as desired under changing drilling load conditions. Furthermore, frequent changes in pressure on the trigger distract the user from holding the drill handle steady for drilling a neat round hole at the desired angle of entry into the workpiece.
Insofar as I am aware, the concept of reducing the energy consumption of induction motors by automatically decreasing the effective voltage being applied to the motor when running no load or light loading conditions is entirely novel.